1. Field of the Invention
The present invention relates to an epitaxial wafer, a method for producing the epitaxial wafer, a semiconductor element, and an optical sensor device. More specifically, the present invention relates to an epitaxial wafer including a multiple quantum well (MQW) structure which is composed of a III-V compound semiconductor and whose bandgap energy corresponds to a near infrared region to a far infrared region, a method for producing the epitaxial wafer, a semiconductor element, and an optical sensor device.
2. Description of the Related Art
InP-based semiconductors, which are composed of III-V compounds, have a bandgap energy corresponding to a near infrared region to an infrared region and hence a large number of studies are conducted to develop photodiodes for communications, image capturing at night, and the like. For example, J. Hu, et. al. “Type II photoluminescence and conduction band offsets of GaAsSb/InGaAs and GaAsSb/InP heterostructures grown by metalorganic vapor phase epitaxy”, APPLIED PHYSICS LETTERS, Vol. 73, No. 19 (1998), pp. 2799-2801 discloses an experiment in which each of InGaAs/GaAsSb type-II MQWs including InGaAs layers with different thicknesses is formed on an InP substrate and the resulting change in the bandgap energy is detected using a photoluminescence emission spectrum. R. Sidhu, et. al. “A Long-Wavelength Photodiode on InP Using Lattice-Matched GaInAs—GaAsSb Type-II Quantum Wells”, IEEE Photonics Technology Letters, Vol. 17, No. 12 (2005), pp. 2715-2717 discloses a photodiode in which an InGaAs/GaAsSb type-II MQW is formed on an InP substrate and a p-n junction is formed by performing doping during the growth of an epitaxial layer, the photodiode having characteristic sensitivity in a wavelength range of 1.7 μm to 2.7 μm. R. Sidhu, et. al. “A 2.3 μm Cutoff Wavelength Photodiode on InP Using Lattice-Matched GaInAs—GaAsSb Type-II Quantum Wells”, 2005 International Conference on Indium Phosphide and Related Materials, pp. 148-151 discloses a photodiode including a type-II MQW absorption layer having 150 pairs layered such that 5 nm InGaAs and 5 nm GaAsSb constitute a single pair, the photodiode having characteristic sensitivity (200 K, 250 K, and 295 K) in a wavelength range of 1 μm to 3 μm.
In addition, Japanese Unexamined Patent Application Publication No. 2011-60853 proposes the following technique: in a photodiode that includes an absorption layer containing antimony (Sb) as a group V element and an InP window layer, a donor impurity is incorporated into the InP window layer and, as a result, incorporation of antimony into the InP window layer causing conversion into a p-type window layer is canceled out to thereby decrease the dark current.
The above-described photodiodes are each disposed on an InP (100) substrate. The surface of the InP (100) substrate is subjected to a typical treatment such as wet etching as a matter of course, but the surface morphology of the InP (100) substrate is not further controlled. However, when a thick epitaxial layered body is formed on a substrate like a photodiode, the surface morphology of an epitaxial layered body after deposition is highly dependent on the surface form of the substrate at the beginning of the deposition. Consequently, the crystallinity of the epitaxial layered body varies in accordance with the surface morphology. That is, when the surface morphology of a top layer of the epitaxial layered body is poor, the crystallinity of a layered body is also poor.